The present invention relates to sampling devices for capturing airborne or liquid-suspended particles. More specifically, the present invention relates to an apparatus for collecting aerosols from wall cavities. As used herein, reference to wall cavities is intended to be given the broadest interpretation including cavities within wall structures, ceiling structures, and the like. 
There are numerous prior art filter sampling devices available today. A typical filtration cassette has an inlet and outlet for directing the flow of an initial sample medium, either liquid or gas. Using a vacuum source, gas and/or liquid molecules, as well as particles, are made to pass through the filter medium. The filter medium selectively retains particles based on the particle's size. 
An additional conventional air sampling device is commercially available under the trade name, Air-O-Cell®. This air sampler includes a two part cassette having a narrowing inlet and an outlet which is also connected to a vacuum source. Between the two parts of the cassette is a slit directing the air flow onto a 2 mm×14 mm impactor plate having an adhesive on the plate's upper side. In operation, spores, pollen, fibers, etc. enter through the inlet, strike the plate and are adhered to its surface. For analysis, the plate is removed and viewed under a microscope.  
Similar to the apparatus described above, European Patent No. 0,129,983 discloses a filter cassette for sampling airborne particles having an inlet, an outlet and a frusto-conically shaped filter medium positioned in between. U.S. Pat. No. 5,437,198 describes a device for separating and capturing airborne particles having a slit nozzle inlet and an internal porous impaction surface. Unimpacted particles follow the airflow past the impaction surface to be either later collected or discharged. 
Unfortunately, none of the above described air collection devices, also referred to as sampling cassettes, are suitable for aerosol sampling within a wall cavity. In particular, such cassettes are too large to conveniently fit inside a wall cavity. To overcome this obstacle, a small diameter sample probe is typically attached to the inlet of the cassette, and the sample probe is inserted through a hole in the wall structure into the wall cavity. The aerosols are withdrawn from the wall cavity, passing through the sample probe and into the cassette where it is collected upon an impactor plate or filter medium for analysis. 
A commercially available system for collecting airborne aerosols from wall cavities is sold under the tradename WallChek®. The WallChek® system includes a circular hood machined from plastic that fits snugly over the inlet of an aerosol sampling cassette. The WallChek® system further includes a plastic tube of small diameter. The proximal end of the tube is attached to the inlet of the hood and the distal end of the plastic tube is fitted with a plastic or rubber cap. A hole is punched or drilled through the wall and the capped end of the flexible tube is inserted through the hole into the wall cavity. Once inserted, a metal rod is  projected through the hole and used to dislodge the protective cap from covering the distal end of the tube. The machined hood is then attached to the inlet of an aerosol sampling cassette. A volume of air is drawn through the cassette by a vacuum source causing sample media to travel through the flexible tube into the sampling cassette. 
Unfortunately, the WallChek® device is extremely expensive, costing several hundred dollars. Because of this high cost, the WallChek® assembly is typically reused, with subsequent samples requiring the use of the same hood and flexible tube assembly. This prior art construction is thus susceptible to contamination from prior sampling. 
In order to reduce contamination, the hood and flexible tube are typically cleaned between sampling procedures. Unfortunately, this is an often difficult task to perform, particularly when sampling is conducted in the field where cleaning facilities are not available. 
An additional disadvantage of the WallChek® system is that the internal diameter of the flexible tube is relatively small causing the tube to be easily clogged by dust and debris generated when the hole is formed in a wall by punching or drilling through drywall or plaster. Though a protective cap is provided when inserting the tube into the wall, the process for removing the cap with a metal rod is cumbersome and time consuming. 
Still an additional disadvantage with the prior art wall cavity sample probe is that it uses a thin flexible plastic tube which has only a single opening at the tip of the tube. The opening of  the tube is prone to becoming blocked if positioned against a solid object within the wall cavity, such as the wall structure at the rear of the wall cavity. Moreover, if the tube encounters an object within the wall cavity during insertion, the tube is prone to bending sharply, causing crimping of the tube and a reduction in the amount of airflow into the sampling cassette which can effect the integrity of the sample. 
It would therefore be advantageous to provide a sample probe for wall cavities which is inexpensive to manufacture, not prone to clogging, and not prone to loss of integrity due to crimping. 
It would also be advantageous to provide a wall sample probe in which clogging by dust and debris is not a practical concern. 
Furthermore, it would be advantageous to provide an improved wall probe which did not require cleaning between field samples. 